IL-33 is a newly identified member of the IL-1 family of cytokines. Administration of recombinant IL-33 has been shown to alter a diverse range of diseases, including atherosclerosis, infection and allergy via its receptor ST2. Endothelial and epithelial cells have been shown to express IL- 33 but in the nucleus where it acts as a transcriptional regulator while we have shown mast cells produce IL-33 during inflammation. As such, IL-33 has been proposed to have duel functions: a immunomodulatory cytokine and a transcriptional regulator. Mast cells are tissue-resident cells and have been shown to be important in controling the generation of immunity in a diverse range of diseases, including autoimmunity, cancer, and allergy. We have discovered that mast cells express IL-33 basally and upregulate expression after activation by crosslinking of IgE by specific antigens. This is highly dependent on calcium, since upregulation was blocked by EDTA and ionomycin was sufficient to induce expression. Unlike endothelial and epithelial cells, we observe IL-33 in both the cytoplasmic and nuclear compartments. We have demonstrated that ST2 and IL-33 are critical for the recruitment of inflammatory cells into the skin during the latephase inflammatory response of anaphylaxis. Based on our preliminary data, we hypothesize that mast cells release IL-33 during activation (extrinisic activity) to promote recruitment and escalation of inflammation, as well as IL-33 altering the transcriptional profile of the mast cell (intrinsic activity) by regulating gene expression. We predict that the expression of IL-33 is highly dependent on calcium and NFAT binding to the proximal promoter region of the il33 gene. These concepts will be addressed by 3 specific aims that utilize both in vitro and in vivo approaches. Specific Aim 1 will test the role of mast cell-derived IL-33 in the generation of tissue inflammation and immunity in models of anaphylaxis. Specific Aim 2 will focus on the effects of IL-33 on regulating mast cell gene expression and chromatin remodeling. Specific Aim 3 will determine the requirements for calcium and NFAT-mediated transcription on the expression of IL-33 in mast cells. Our study will define the mechanisms through which mast cell-derived IL-33 modulates inflammation.